Products containing light emitting diodes (LEDs) or lasers that emit light in the visible or infrared range must conform to eye safety requirements, IEC 60825-1. If the optical flux that can impinge on a user's eye exceeds the standard, the device must be labeled an eye safety hazard, which is undesirable. This requirement applies not only during normal operation of the circuit but when predictable single faults occur in the circuit.
FIG. 1 illustrates a prior art eye safety circuit. This simple circuit uses a fuse. When the bias current exceeds the eye safety requirement, the fuse blows. FIG. 2 illustrates another prior art eye safety circuit. This circuit is a retriggerable transistor circuit that shunts current away from the LED when the current exceeds a design threshold.
The prior art circuits detect when the LED current is higher than some preset current. However, the circuit in FIG. 1 cannot detect a fault in which the LED cathode is connected to GROUND, and the fuse cannot be readily integrated onto an integrated circuit. The circuit in FIG. 2 can detect a fault where the LED cathode is shorted to Ground, but this circuit would be difficult to implement on a standard CMOS integrated circuit process.
As shown in FIG. 3, Kinoshita in “Semiconductor Laser Driving Device for Stabilizing the Optical Output Thereof”, U.S. Pat. No. 4,884,280, issued 28 Nov. 1989, disclosed using a monitoring photodiode having an output indicative of the actual output of the laser. A comparator compares a reference signal indicative of a reference output level of the laser and the monitor photodiode signal. The comparator output signal reflects the change in the actual optical output level of the laser. An abnormal current eliminator controls the comparator output so that the signal has limited amplitude. Hence, the drive current to the laser is forcibly decreased to safe drive current levels.
As shown in FIG. 4, Stafford, in “Fault Detection in a LED Bias Circuit”, U.S. Pat. No. 6,704,183, issued 9 Mar. 2004, assigned to Agilent Technologies, discloses using comparators at different points in the circuit. However, a small leakage path to Ground potential from the light emitting diode cathode will not be detected. This leakage current can be enough to cause the light emitting diode to operate outside of the eye safety limits. In addition, the light emitting diode must be active during the probing which can pose a safety hazard if excessive current does exist.